The following problems will be investigated: (1) We discovered recently that heterozygous rabbits of the allotype al, 3/b4 injected with As-TMA-BSA (As equals azophenylarsonate; TMA equals azophenyl-N- trimethylammonium; BSA equals bovine serum albumin) produce anti-As antibodies predominantly or exclusively of the all allotype, whereas the anti-TMA and anti-BSA antibodies are expressed in approximately equal amounts as al and a3 allotypes. We intend to find out whether the predominance in anti-As antibodies of the al allotype involves a decrease in the total amount of anti-As antibodies. (2) Since the amino acid composition of the heavy chains of allotype al and a3 is different, we will try to separate the two allotypic chains by ion exchange chromatography, by gel filtration or by a combination of both. (3) We intend to determine cystine and tryptophan in anti-As and anti-TMA antibodies produced in rabbits injected with As-TMA-BSA. (4) We continue investigating the reasons for the heterogeneity of the light chains of normal rabbit immunoglobulins by injecting the animals with radioactive leucine and determining the specific radioactivity of the isolated bands at different periods of time after the injection.